This invention relates to a low temperature sealing composition, and more particularly, to a low temperature sealing glass composition suitable for hermetically sealing ceramic packages for integrated circuits (ICs) and quartz resonators, and sealing display panels such as vacuum fluorescent displays (VFDs).
A sealing material for hermetically sealing packages for an IC or quartz resonators, and display panels are required to be sealable at a low temperature so as not to deteriorate the enclosed devices due to thermal effects. In addition, it is also necessary that the thermal coefficient of expansion (hereinafter, abbreviated to TCE) of the material is compatible with that of the ceramics or a display panel glass to be used. In particular, the IC packages should be sealed with the material that is high in mechanical strength besides meeting the aforementioned requirements. The material for sealing the IC package is also required to have superior insulating properties to avoid undesirable leak of signal currents. Further, the material for the IC package should have the least possible content of .alpha.-particle emitting impurities because the soft error often occurs when an IC chip such as DRAM and CCD is exposed to the .alpha.-particle.
Various sealing materials have been proposed that meet the above mentioned requirements. One of such materials is a PbO-B.sub.2 O.sub.3 glass of a low melting temperature. It has also been proposed to use the low melting temperature glass as an admixture with refractory fillers. For example, Japanese Patent Unexamined Publication 229738/1990 discloses a sealing material comprising PbO-B.sub.2 O.sub.3 glass powder, lead titanate ceramic powder and low expansion ceramic powder.
It is known about the PbO-B.sub.2 O.sub.3 glass that lowering content of B.sub.2 O.sub.3 generally lowers the glass transformation point resulting in a lower sealing temperature. However, sufficient content of B.sub.2 O.sub.3 is necessary to make the glass stable, which limites glass composition and makes it impossible to obtain sealing materials usable below 400.degree. C. The sealing temperature of 400.degree. C. or lower is thus difficult to be achieved. This means that the sealing material of the type described is not suitable for sealing packages for heat sensitive devices such as a highly integrated IC, or quartz resonators.
Several sealing materials have also been proposed which are sealable at the temperature of 400.degree. C. or lower. For example, Japanese Patent Publication No. 8060/1988 discloses a sealing material consisting of PbO-B.sub.2 O.sub.3 glass powder with Tl.sub.2 O contained therein. In addition, U.S. Pat. No. 4,743,302 discloses another sealing material comprising PbO-V.sub.2 O.sub.5 -TeO.sub.2 glass powder. However, both of these materials are unfavorable because of the following reasons. The sealing material disclosed in the Japanese Patent Publication No. 8060/1988 includes a highly toxic component, i.e., thallium carbonate in the raw material for the glass. Such a toxic material should be handled using special protective instrument. The sealing material disclosed in U.S. Pat. No. 4,743,302 is more likely to be crystallized and thus requires application of a large load to the package to be sealed.